The present invention relates to the art of diagnostic imaging. It finds particular application in conjunction with nuclear or gamma cameras and will be described with particular reference thereto. It is to be appreciated, however, that the present invention will also find application in other non-invasive investigation techniques and imaging systems such as single photon planar imaging, whole body nuclear scans, positron emission tomography (PET), digital x-ray computed tomography and other diagnostic modes. It is to be further appreciated that the present invention will also find application in other Compton-based systems such as Compton-type telescopes used for astronomy.
Single photon emission computed tomography (SPECT) has been used to study a radionuclide distribution in a subject. Typically, one or more radiopharmaceuticals or radioisotopes are injected into a patient subject. The radioisotope preferably travels to an organ of interest whose image is to be produced. The patient is placed in an examination region of the SPECT system surrounded by large area planar radiation detectors. Radiation emitted from the patient is detected by the radiation detectors. The detectors have a mechanical collimator to limit the detector to seeing radiation from a single selected trajectory or ray, often the ray normal to the detector plane.
Typically, the detector includes a scintillation crystal that is viewed by an array of photomultiplier tubes. The relative outputs of the photomultiplier tubes are processed and corrected, as is conventional in the art, to generate an output signal indicative of (1) a position coordinate on the detector head at which each radiation event is received, and (2) an energy of each event. The energy is used to differentiate between emission and transmission radiation and between multiple emission radiation sources and to eliminate stray and secondary emission radiation. A two-dimensional projection image representation is defined by the number of radiation events received at each coordinate.
The mechanical collimator used in conventional gamma cameras, such as an Anger camera, localize the gamma emitters. This type of collimator, however, leads to low efficiency because only a fraction of the radiation passes through the collimator. Furthermore at any given time, only one view of an object of interest is obtained. Thus, the camera needs to move or rotate relative to a subject in order to collect all the data necessary for image reconstruction. Further, the collimators are fabricated of lead. Typically, each collimator is of sufficient weight that it must be connected to and removed from the head by mechanical, rather than human means. Not only is handling inconvenient, but the supporting structure for the detectors must support the detector head and hundreds of kilograms of collimator stably and without vibration.
A new type of gamma camera for SPECT relies on Compton scattering for gamma source localization and is known as a Compton camera. This camera has been proposed as an alternative to the conventional Anger camera and is advantageous because it uses electronic rather than mechanical collimation. Electronic collimation provides both high geometric efficiency and multiple image views. A proposed example of image reconstruction from data collected by a Compton camera is described in "Towards direct reconstruction from a gamma camera base on Compton scattering," by M. J. Cree and P. J. Bones, IEEE Trans. Med. Imag., Vol. 13, pp. 398-407, 1994. Although some progress has been made toward image reconstruction from a Compton camera system, at present, an acceptable filtered backprojection algorithm has proved elusive.
The present invention provides a new and improved reconstruction algorithm for a Compton camera which overcomes the above-referenced problems and others.